Coated metal food container article

ABSTRACT

The article provides a coating that is particularly resistant to absorption of flavor producing compounds such as limonene, thereby providing a more acceptable flavor profile interaction than state-of-the art polyolefin coated metals.

The present invention relates to a coated metal food container article.Cans used to contain consumable food products, for example, carbonatedsoft drinks (CSDs), need to be coated to keep the contained food frommetal pickup and to protect the can from oxidation. Consequently, thecans are coated to provide a barrier coating to protect both the foodand the can. Current coatings, which are often epoxy coatings derivedfrom bisphenol A, have flavor interactions with the food product thatare well-accepted by consumers and manufacturers. However, continued useof materials made from bisphenol A in consumer products has come underscrutiny by various agencies and governments, thereby encouraging thesearch for an acceptable alternative coating material. One suchalternative material is a film made from a polyolefin dispersion, suchas described in U.S. Pat. No. 8,779,053. One of the challenges withstate-of-the-art films prepared from polyolefin dispersions is thatthese films interact with the can and the food product differently fromthe incumbent films. Therefore, it would be an advance in the art of cancoatings to discover a film that more closely matches the interactionsof the incumbent resin material.

BACKGROUND OF THE INVENTION Summary of the Invention

The present invention addresses a need in the art by providing anarticle comprising a cured polymeric film superposing a metal, whereinthe cured polymeric film comprises:

a) a base polymer comprising structural units of ethylene and aC₁-C₄-alkyl acrylate or methacrylate, wherein the weight-to-weight ratioof the structural units of ethylene and the C₁-C₄-alkyl acrylate ormethacrylate is in the range of from 99.8:0.2 to 50:50; andb) a dispersant which is a copolymer comprising structural units ofethylene and a carboxylic acid monomer, wherein the copolymer has a meltflow index in the range of from 50 to 2000 g/10 min at 190° C., and theweight-to-weight ratio of structural units of ethylene to carboxylicacid monomer is in the range of from 95:5 to 70:30;wherein the dispersant has a concentration in the range of from 9 to 50weight percent based on the weight of the cured film; wherein the sum ofthe dispersant and the base polymer comprise from 10 to 100 percent ofthe weight of cured film; and wherein the film has a thickness in therange of from 0.5 μm to 25 μm. The article of the present inventionaddresses a need by providing a coated metal that gives a moreacceptable flavor profile interaction than state-of-the art polyolefincoated metals.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is an article comprising a cured polymeric filmsuperposing a metal, wherein the cured polymeric film comprises:

a) a base polymer comprising structural units of ethylene and aC₁-C₄-alkyl acrylate or methacrylate, wherein the weight-to-weight ratioof the structural units of ethylene and the C₁-C₄-alkyl acrylate ormethacrylate is in the range of from 99.8:0.2 to 50:50; andb) a dispersant which is a copolymer comprising structural units ofethylene and a carboxylic acid monomer, wherein the copolymer has a meltflow index in the range of from 50 to 2000 g/10 min at 190° C., and theweight-to-weight ratio of structural units of ethylene to carboxylicacid monomer is in the range of from 95:5 to 70:30;wherein the dispersant has a concentration in the range of from 9 to 50weight percent based on the weight of the cured film; wherein the sum ofthe dispersant and the base polymer comprise from 10 to 100 percent ofthe weight of cured film; and wherein the film has a thickness in therange of from 0.5 μm to 25 μm. The article of the present inventionaddresses a need by providing a coated metal that gives a moreacceptable flavor profile interaction than state-of-the art polyolefincoated metals.

The polymeric film is conveniently formed by applying an aqueousdispersion comprising a dispersant, a base polymer, and a neutralizingagent, which dispersion may be prepared by a continuous or batchprocess. An example of a preferred continuous process is twin screwextrusion, as described in U.S. Pat. No. 8,722,787, Comparative ExampleE. Alternatively, a batch process using, for example, with a 2CVHelicone mixer, which is a conical batch mixer that uses dualintermeshing conical blades to mix high viscosity materials. Theconcentration of polymer solids in the aqueous dispersion is preferablyin the range from 20, more preferably from 25, and most preferably from30 weight percent, to preferably 50 and more preferably to 45 weightpercent, based on the concentration of water and base polymer.

The dispersant is a copolymer comprising structural units of ethyleneand a carboxylic acid monomer such as acrylic acid, methacrylic acid, oritaconic acid. As used herein, the term “structural unit” of the namedmonomer refers to the remnant of the monomer after polymerization. Forexample, a structural unit of methyl methacrylate is as illustrated:

where the dotted lines represent the points of attachment of thestructural unit to the polymer backbone.

The dispersant has a melt flow index in the range of from 50 to 2000g/10 min at 190° C. (according to ASTM D1238) and the weight-to-weightratio of structural units of ethylene to carboxylic acid monomer is inthe range of from 95:5, preferably from 90:10, and more preferably from85:15 weight percent; to 70:30, and preferably to 75:25 weight percent,based on the weight of the copolymer. The concentration of the copolymeris preferably in the range of from 10, more preferably from 15 weightpercent, to preferably to 40, more preferably to 30 weight percent,based on the weight of the cured film. Examples of suitable commerciallyavailable dispersants include Primacor 5980i and NUCREL™ 2806 Copolymer(A Trademark of The Dow Chemical Company or its Affiliates).

The base polymer is a copolymer comprising structural units of ethyleneand a C₁-C₄-alkyl acrylate or methacrylate, wherein the weight-to-weightratio of the structural units of ethylene to the C₁-C₄-alkyl acrylate ormethacrylate is in the range of from 99.8:0.2, preferably from 99.7:0.3;and more preferably from 99.6:0.4; to 50:50, more preferably to 60:40;and most preferably to 65:35. Preferred base polymers includeethylene-co-methyl acrylate; ethylene-co-ethyl acrylate; andethylene-co-butyl acrylate. Preferably, the concentration of the basepolymer is preferably in the range of from 1, more preferably from 2, to90, more preferably to 80, and most preferably to 75 weight percent,based on the weight of the cured film. Commercial examples of basepolymers include: AMPLIFY™ EA103 Functional Polymer, ELVALOY™ AC34035Ethylene butyl acrylate copolymer; and AC1609 Ethylene methyl acrylatecopolymer. (AMPLIFY and ELVALOY are Trademarks of The Dow ChemicalCompany or its Affiliates.) The dispersant used to prepare the aqueousdispersion is advantageously neutralized with an organic base having aboiling point of less than 250° C., preferably ammonia or an amine;examples of suitable amines such as N,N-dimethylethanolamine,diethylamine, and morpholine.

The concentration of neutralizing agent is sufficiently high toneutralize at least half of the carboxylic acid groups in thedispersant. For example, if the dispersant comprises 0.05 mol ofcarboxylic acid groups, at least 0.025 mol of an amine such asN,N-dimethylethanolamine would be required. Thus, the ratio of basefunctionality in the neutralizing agent, preferably amine groups orammonia, to carboxylic acid groups in the dispersant is at least 0.5:1.Preferably the ratio is in the range of from 0.7:1, more preferably from0.9:1, more preferably from 1.1:1, and most preferably to 1.3:1; topreferably 3:1, more preferably to 2.5:1; and most preferably to 2.0:1.

The composition used to make the film may comprise other componentsincluding polymeric coupling agents to improve the compatibility betweenthe dispersant and the base polymer. Examples of a suitable couplingagent include ethylene-co-maleic anhydride and polyolefin grafted withmaleic anhydride, which, when used, is preferably present at aconcentration in the range of from 5, more preferably from 10 weightpercent to preferably 65, more preferably to 50, and most preferably to30 weight percent based on the weight the cured film.

The composition used to make the film may further comprisenon-functionalized ethylene polymers and copolymers such as high densitypolyethylene, as well as ethylene-co-alkene copolymers such asethylene-co-propylene, ethylene-co-hexene, or ethylene-co-octenecopolymers. These non-functionalized copolymers may be used at aconcentration of up to 85 weight percent, based on the weight of thecured film. In one embodiment, the composition used to make the filmcomprises from 20 to 50 weight percent non-functionalized ethylenepolymers or copolymers based on the weight of the cured film.

The composition may also comprise up to 5 weight percent, based on theweight of the cured film, of a wax such as ethylene bis(stearamide) andpolyolefin waxes such as the commercially available Polywax 655Polyethylene.

The composition can be applied to a metal substrate, for example, usinga wire wound drawdown bar. The wet film can then heated to remove water,preferably to a temperature in the range of from 50° C., more preferablyfrom 70° C. to preferably 250° C., more preferably 220° C. to provide acoat thickness in the range of from 0.5 μm, preferably from 1 μm, andmore preferably from 2 μm to 25 μm, preferably to 15 μm, and morepreferably to 10 μm. It is desirable in practice to remove as muchneutralizing agent as possible during the heating (curing) process.Examples of preferred metals include aluminum, steel, and tin-platedsteel.

Other suitable means for applying the composition to the metal substrateinclude spray coating and roller coating techniques well known in theart.

It has been discovered that a metal substrate coated with a polyolefinfilm as described hereinabove provides a more acceptable flavor profileinteraction than state-of-the art polyolefin coated metals, asdemonstrated by the below examples.

In the following examples and comparative example, EA103 refers toAMPLIFY™ EA103 Ethylene Ethyl Acrylate Polymer; 5980i refers to Primacor5980i Copolymer; DMEA refers to dimethylethanolamine; MA-g-PE refers toLicocene 431 maleic anhydride grafted polyethylene wax; 8401 refers toENGAGE™ 8401 Polyolefin Elastomer; 8402 refers to ENGAGE™ 8402Polyolefin Elastomer; AC34035 refers to ELVALOY™ AC34035 Ethylene ButylAcrylate Copolymer; AC1609 refers to ELVALOY™ AC1609 Ethylene MethylAcrylate Copolymer, melt flow index=6, 9% acrylate; and AC12024S refersto ELVALOY™ AC12024S Ethylene Methyl Acrylate Copolymer, melt flowindex=20, 24% acrylate. (ELVALOY and ENGAGE are Trademarks of The DowChemical Company or its Affiliates.)

EXAMPLES Example 1—Preparation of an Aqueous Dispersion of Base Polymer,Dispersant, and Polymeric Coupling Agent at a 65:25:10 w/w/w Ratio

The melt chamber of a 2CV Helicone mixer was preheated to 90° C. andthen loaded with AC34035 (52.02 g), MA-g-PE polymeric coupling agent(8.00 g), and 5980i (20.02 g), for a mix composition of 65.0% AC34035,10.0% MA-g-PE, and 25.0% 5980i. Deionized water (22.83 mL) and DMEA(12.71 mL, 200% neutralization) were also pre-loaded using an ISCOsyringe pump. The components in the melt chamber were heated 143° C., atwhich temperature the mixer was started; the initial mixing speed wasmaintained at 43 rpm for 5 min then raised to 98 rpm for the remainderof the run. The material was white and uniform after 30 min of mixing.Dilution water was added with an ISCO pump at the rate of 1 mL/min for30 min, then 1.5 mL/min for 56 min. The dilution was paused for 8 minduring the second part of the addition. After the dilution was complete,the mixer was turned off and the contents were cooled. Once thetemperature of the contents reached 88.6° C., the pressure in thechamber was slowly vented. The gate valve was opened and the materialwas collected (200.83 g, 88.01% recovery) and filtered before use.

All samples were prepared by formulating an aqueous polyolefindispersion with a 0.69:1 w/w organic solid to solvent ratio, wherein thesolvent was a 1:1 w/w mixture of n-butanol and Butyl CELLOSOLVE™Ethylene Glycol Monobutyl Ether (A Trademark of The Dow Chemical Companyor Its Affiliates). The formulation also contained Primid QM-1260hydroxalkyl amide at a ratio of 0.42 equivalents of hydroxyl groups peracid groups of the aqueous dispersion. The final solid content wasadjusted to 20 weight percent by addition of water containing 0.3 weightpercent of neutralizing agent used to make the dispersion. Theseformulation were coated on both sides of aluminum coupons using a #16wire wound draw-down bar. The films were then cured in a Despatch ovenat 188° C. for 4 min for each side. Film thicknesses were measured usinga Byko-Test MPOR.

Example 2—Preparation of an Aqueous Dispersion of Base Polymer,Dispersant, and Polymeric Coupling Agent at a 65:25:10 w/w/w Ratio

This Example was prepared substantially as described in Example 1 exceptthat AC1609 (51.99 g) was used in place of AC34035. The dispersion(188.90 g, 82.8% recovery) was collected and the material was filteredbefore use.

Example 3—Preparation of an Aqueous Dispersion of Base Polymer,Dispersant, Polymeric Coupling Agent, and 8401 at a 18:25:10:47 w/w/w/wRatio

EA103 (18 weight percent of polymer solids), 5980i (25 weight percent ofpolymer solids), MA-g-PE (10 weight percent of polymer solids), and 8401(47 weight percent of polymer solids) were fed individually andconcurrently from separate hoppers at the specified relative weights ata rate of 40 lbs polymer/h (18.1 kg/h) into a 25-mm Bersdorff ZE25 UTXextruder with 48 L/D. The extruder temperature profile was ramped to180° C. prior to the introduction, through ISCO pumps, of water (72.7mL/min) and DMEA (33.5 mL/min) separately and concurrently. Dilutionwater was then added to the extruded to adjust solids concentration to˜43 weight percent; the mixture was then cooled using a heat exchangerprior to exit. A back-pressure regulator was used at the extruder outletto adjust the pressure in the extruder barrel.

Example 4—Preparation of an Aqueous Dispersion of Base Polymer,Dispersant, and Polymeric Coupling Agent at a 65:25:10 w/w/w Ratio

This Example was prepared substantially as described in Example 1 exceptthat AC12024S (51.99 g) was used in place of AC34035. The dispersion(158.53 g, 69.5% recovery) was collected and the material was filteredbefore use.

Comparative Example—Aqueous Dispersion Containing Polyolefin Particlesnot Functionalized with Structural Units of a C₁-C₄-Alkyl Acrylate

CANVERA™ 1110 Polyolefin Dispersion (43 weight percent solids, ATrademark of The Dow Chemical Company or its Affiliates), is notfunctionalized with structural units of a C₁-C₄-alkyl Acrylate was usedas a comparative against the examples of the present invention, all ofwhich describe the preparation of aqueous dispersions of C₁-C₄-alkylacrylate functionalized polyolefin particles.

Incubation and Extraction:

Samples were incubated in Coca Cola, 3 aluminum panels per glass jar,completely submerged for 7 d at 30° C. After incubation was complete,the panels were rinsed with DI water and then extracted using 15 mLmethylene chloride per panel. Octadecane (5 ppm) was used as an internalreference standard.

Method for Determining Limonene Absorption

Absorption of limonene, a prominent contributor to the flavor of CocaCola, was chosen a proxy for flavor absorption by the polyolefincoating. Absorption was measured by gas chromatography using an Agilent7890 Series GC.

Table 1 shows Limonene absorption/μm (A/μm) in the aluminum panelssubmerged in Coca Cola.

Table 1 Limonene Absorption in Al Panels Film Ex No. Thickness (μm)Comonomer A/μm 1 3.6 Butyl Acrylate 1.68 2 5.6 Methyl Acrylate 0.77 35.6 Ethyl Acrylate 0.66 4 4.8 Methyl Acrylate 1.07 Comp. 1 5.1 none 2.64

The results show that coatings prepared from an aqueous dispersion of apolyethylene functionalized with a C₁-C₄-alkyl acrylate shows superiorresistance to absorption to the panels than the coating prepared fromthe aqueous dispersion of the unfunctionalized polyethylene. Coatingsprepared from ethylene methyl acrylate and ethylene ethyl acrylate wereespecially resistant to absorption.

1. An article comprising a cured polymeric film superposing a metal,wherein the cured polymeric film comprises: a) a base polymer comprisingstructural units of ethylene and a C₁-C₄-alkyl acrylate or methacrylate,wherein the weight-to-weight ratio of the structural units of ethyleneand the C₁-C₄-alkyl acrylate or methacrylate is in the range of from99.8:0.2 to 50:50; and b) a dispersant which is a copolymer comprisingstructural units of ethylene and a carboxylic acid monomer, wherein thecopolymer has a melt flow index in the range of from 50 to 2000 g/10 minat 190° C., and the weight-to-weight ratio of structural units ofethylene to carboxylic acid monomer is in the range of from 95:5 to70:30; wherein the dispersant has a concentration in the range of from 9to 50 weight percent based on the weight of the cured film; wherein thesum of the dispersant and the base polymer comprise from 10 to 100percent of the weight of cured film; and wherein the film has athickness in the range of from 0.5 μm to 25 μm.
 2. The article of claim1 wherein the cured film has a thickness in the range of from 1 μm to 15μm; wherein the dispersant is a copolymer of ethylene and acrylic acidor methacrylic acid.
 3. The article of claim 2 wherein the cured filmhas a thickness in the range of from 2 μm to 10 μm; wherein thecarboxylic acid monomer is acrylic acid or methacrylic acid; wherein theconcentration of structural units of ethylene to structural units ofacrylic acid or methacrylic acid is in the range of from 90:10 to 75:25;and the concentration of the dispersant is in the range of from 10 to 40percent based on the weight of the cured film.
 4. The article of claim 3wherein the weight-to-weight ratio of the structural units of ethyleneand the C₁-C₄-alkyl acrylate or methacrylate is in the range of from99.6:0.4 to 60:40; wherein the concentration of the base polymer is from2 to 75 weight percent based on the weight of the cured film; andwherein the base polymer comprising structural units of ethylene and theC₁-C₄-alkyl acrylate or methacrylate is ethylene-co-methyl acrylate,ethylene-co-ethyl acrylate, or ethylene-co-butyl acrylate.
 5. Thearticle of claim 1 wherein the cured film further comprises from 5 to 65weight percent, based on the weight of the cured film,ethylene-co-maleic anhydride or ethylene grafted maleic anhydride or acombination thereof; and wherein the base polymer comprising structuralunits of ethylene and the C₁-C₄-alkyl acrylate or methacrylate isethylene-co-methyl acrylate or ethylene-co-ethyl acrylate.
 6. Thearticle of claim 5 which wherein the cured film further comprises from20 to 50 weight percent, based on the weight of the film, of anethylene-co-propylene, an ethylene-co-hexene, or an ethylene-co-octenecopolymer, or a high density polyethylene, or a combination thereof. 7.The article of claim 6 which wherein the cured film further comprises upto 5 weight percent, based the weight of the cured film, of a wax.